1. Field of the Invention
The present invention relates to thermal insulation materials that are based on aerogels, a process for manufacturing these, and the use thereof.
2. Description of the Related Art
Aerogels are highly porous, low-density materials that are manufactured by forming a gel and then removing the liquid therefrom, whilst retaining the gel structure to a large extent when this is done.
According to a narrow definition (for example, see Gesser and Goswanni, Chemistry, rev. 1989, 89, 767), aerogels are materials in which the liquid is removed from the gel under super-critical conditions, whereas if the gel is dried under sub-critical conditions one refers to xerogels, and in the event that the liquid is removed from the frozen state by sublimation, one speaks of cryogels.
Within the context of the present intention, aerogels include all these materials and can contain any other gases apart from air. The gases can also be removed from the aerogel in a vacuum.
In particular, aerogels with porosities of greater than 60% and densities of less than 0.6 g/cm.sup.3 exhibit very low thermal conductivities and for this reason are well suited for thermal insulation materials. Because of their low density, such materials exhibit little mechanical stability, however.
Formulations containing aerogels that also incorporate a binder in order to overcome this disadvantage are described, for example, in EP-A 0 340 707 and DE-A 44 37 424. In order to obtain highly effective aerogels that can additionally be transparent, the aerogel can be reinforced by incorporating fibres (see, for example, DE-A 44 30 642). However, all of these processes entail the disadvantage that the incorporation of alien substances increases the thermal conductivity of the aerogel material.
Brinker et al. (Nature, 1995, 374, 439) describe a process for manufacturing thin aerogel films on rigid substrates, with thermally insulating cladding for buildings being seen as one possible application.
Surprisingly, it was found that aerogels can be applied to flexible films as a thin layer, which means that aerogel materials of good mechanical stability can be manufactured.
DE-A 38 44 003 describes films that are coated with silica gel, said films additionally containing photochromic compounds. In order to manufactured these, a washing liquid that contains up to 5% polyvinylalcohol in addition to the photochromic substances is placed in a liquor tank. A transparent film, for example of polyacrylate film, is passed through the liquor, and waterglass and sulphuric acid solution is sprayed onto the film of liquid that is formed by doing this, when the silica gel is precipitated out. The film that is so coated is dried at approximately 80.degree. C. at a pressure of 0.1 to 0.5 bar. However, this manufacturing process does not produce a product with a gel structure and for this reason this does not involve an aerogel-coated film.